An experimental study of a hydrogen-containing jet’s impact on a palladium-based catalyst in an air atmosphere was carried out. High-intensity temperature fluctuations on the catalyst surface are obtained in the case when large-scale vortex structures are contained in the jet. These superstructures have a longitudinal size of 20–30 initial jet diameters and a transverse size of about 3–4 diameters. To form such structures, it is necessary to use long, round tubes in the Reynolds number range of 2000–3000 as a source of the impinging jet when a laminar-turbulent transition occurs in the channel according to the intermittency scenario. This effect was obtained at a low hydrogen content in the mixture (XH2 = 3…15%) and a low initial temperature of the catalyst (180 °C). It is shown that the smallest temperature fluctuations are obtained for the laminar flow in the tube (<1.5%), and they are more significant (<4%) for the turbulent regime at low Reynolds numbers (Re < 6000). The greatest temperature fluctuations were obtained during the laminar-turbulent transition in the tube (up to 11%). Two important modes have been established: the first with maximum temperature fluctuations in the local region of the stagnation point, and the second with the greatest integral increase in temperature fluctuations over the entire area of the catalyst.

Here, we present the experimental results on the influence of a detached rib behind a backward-facing step on the flow dynamics and heat transfer. The height of a slot between the lower edge of the rib and the channel wall was varied in the range dh/H = 0.7–1.3, and the distance between the rib and the step was varied within t/H = 0.2–3.2. The rib height was constant and equal to 0.3H, where H is the step height. The fields of static pressures were measured behind the point of flow separation at the step edge. The study was performed in the range of Reynolds numbers 14,200-42,500. The two-dimensional fields of velocities and their fluctuations were measured using the PIV method. Heat transfer was studied in the regime of constant heat flux on the channel surface, where the step was located. Behaviors of the pressure and velocity profiles, as well as heat transfer with varying the detached rib location relative to the step are shown.

An experimental study of spatially localized very large-scale motion superstructures, propagating in a jet of carbon dioxide at low Reynolds numbers, was carried out. A hot-wire anemometer and a high-speed 2D PIV with a frequency of 7 kHz were used as measuring instruments. Such a puff-type superstructure in a jet with a longitudinal dimension of up to 20–30 nozzle diameters are initially formed in the jet source—a long tube in a laminar-turbulent transition mode (without artificial disturbances). It is shown that this regime with intermittency in time, when part of the time flow is laminar and the other part of time is turbulent, exists both at the exit from the nozzle and in the near field of the jet. Thus, the structural stability of such turbulent superstructures in the near field of the jet was found. Despite the large longitudinal scale, these formations have a transverse dimension of the order of several nozzle diameters. These structures have a complex internal topology, that is, superstructures are a conglomeration of vortices of different sizes from macroscale to microscale. Using the example of diffusion combustion of methane in air, it is demonstrated that in reacting jets, the existence of such large localized perturbations is a powerful physical mechanism for a global change in the flame topology. At the same time, the presence of a cascade of vortices of different sizes in the puff composition can lead to fractal deformation of the flame front.

An experimental investigation of a laminar-turbulent transition in a round jet flowing from a cylindrical tube with a diameter of 3.2 mm have been carried out. The range of Reynolds numbers in the experiments Re = Ud / v were of 700-12000. The measurements have been carried out via the PIV system. The profiles of average velocities and their pulsations in the laminar-turbulent transition zone have been obtained, as well as axial distributions of the longitudinal velocity and longitudinal velocity pulsations. Based on a comparison with the data of other authors, the effect of the initial conditions on the laminar-turbulent transition in a submerged jet has been shown.

This paper deals with the experimental study of the gas flow structure at longitudinal flow around a honeycomb surface with hexagonal cells of various sizes and depths. The measurements were carried out using the PIV system for the developed flow in a flat channel with the cross-section 21 × 150 mm and length of 1000 mm. The flow was visualized and the velocity and turbulence components were measured in a smooth channel and in the presence of honeycomb surfaces. It is shown that a boundary layer with high gradient of velocity and turbulent pulsations is formed near the honeycomb.

The laminar-turbulent transition in a round jet flowing from a cylindrical channel with the diameter of 3.2 mm was studied experimentally. In experiments, the range of Reynolds numbers determined by the mean-flow velocity was Re = Ud/v = 700-12000. The measurements were carried out using a PIV system and one-component hot-wire anemometer. The profiles of average velocities and their pulsations in the zone of laminar-turbulent transition were obtained, as well as axial distributions of longitudinal velocity and pulsations of longitudinal velocity.

This paper presents the results of the comprehensive experimental study of flow and heat transfer of the annular impinging jets. The flow fields were studied using the PIV. To measure heat transfer, the miniature gradient sensors of the heat flux were used under the thermal boundary conditions T-w = const. The experimental conditions were as follows: basic round nozzle had diameter d(0) = 17.8 mm; ring nozzles have the same outer diameter and different inner diameters d(2) = 12.7 and 9.1 mm. Air was the working medium. The distance between the nozzle and wall (S/d(0) = 2, 4 and 6) and air flow rate (Reynolds number for the round nozzle was varied within Re = U(0)d(0)/nu(0) = 1.2 x 10(4) - 3.3 x 10(4)) were varied in experiments. Data obtained for the annular and round jets were compared at the same mass air flow rates. It is shown that replacement of the round nozzle by the annular nozzle leads to an increase in intensity of velocity fluctuations in the axial zone of the jet and heat transfer enhancement in the frontal zone of the obstacle. At that, the value of intensification effect depends on the parameters, characterizing the system.

The aim of this work is an experimental study of the flow in subsonic free and impinging macro- and microjets of air. Complex measurements have been performed using the PIV method (Particle Image Velocimetry), hot-wire anemometry and flow visualization. In the experiments, we used the axisymmetric and flat micronozzles with the characteristic size of 250−600 µm. The measurement performed for the jets of the larger size of 1−8 mm are presented for comparison and generalization of experimental data. Air, whose thermodynamic parameters are close to the atmospheric conditions, was used as the working gas. Flow visualization allowed us to construct the spatial pattern of formation and decay of the laminar part of the jet and transition to turbulence. It was shown that the range of a free jet can equals 100−200 nozzle sizes. The Reynolds number of transition to turbulence in mini- and microjets takes high values (1000−2600), and this is two-three tens times higher than the Reynolds number of stability loss. In the impinging jet, an obstacle can change the coordinate of transition to turbulence, for instance, due to the feedback effect.

This work is aimed at experimental investigation of the single rib effect on the recirculation region behind the backfacing step. Measurements were carried out by the PIV method. The experiments were performed at Reynolds number Re = 15000 in the rectangular working channel of 21 X 150 mm and length of 1 m. At the distance of 600 mm from the channel inlet there was the back-facing step of the constant height H=9 mm. Rib height d and distance to the back-facing step S were varied (d from 3 to 6 mm and S from 0 to 77 mm). According to measurements, in this case the following interaction mechanism between separated flows occurs: if there is a rib in the channel in front of the step, the boundary layer separates. Depending on the distance between the rib and the step in the channel the flow will attach in front of the step or behind it. In our experiments the flow attached at S/D >7.5 and Δ=3mm and S/Δ > 10 at Δ= 6mm. In the case when the flow attaches in front of the step, the recirculation zone behind the step decreases to 35%. This relates to the fact that the flow separated behind a rib influences the mixing layer behind the step and intensifies the mixing processes, what finally leads to reduction in the length of recirculation zone. When S/Δ < 7.5, the recirculation zone behind the back-facing step increases, and in the limit case of S/Δ = 0 (the rib is at the step edge) becomes 60% larger for Δ= 3mm. We should note the general tendency of interaction between the separated flows of different scales. A rib changes the typical profile of longitudinal average velocities and mean-root velocity pulsations behind the back-facing step. For the studied configurations it was not enough time for velocity profiles and profiles of velocity pulsations to be restored to the values, corresponding to the separated flow behind the step without a rib. Maximal perturbations within the initial region of separated flow development behind the step were observed at the distance from the wall, corresponding to the coordinate of rib top. In general, the experimental data agree with experimental results of Miau et al., 1991 for S/Δ=12 and calculation results of Neumann, Wengle, 2004. The mechanism of interference for the separated flows of different scales is discussed in detail in the current study.

The current research is aimed at experimental investigation of round and plane submerged macro- and microjets. The measurements were carried out by two methods: Particle Image Velocimetry (PIV) and measurements with the use of hot-wire anemometer. At the first stage there were the experiments on flow visualization in microjets by means of the PIV system. The axisymmetrical microjets were formed with the help of round metal channels with diameter d=500-8000 μm and length of 100 d. The plane channel was formed by two glass plates of 16x70 mm; its height was b =600 μm. The range of Reynolds numbers was Re=Ud/ν=200-6000. It is accepted that submerged jets are significantly unstable, therefore, the Re numbers of laminar-turbulent transition are not higher than 10. However, according to visualization, the jet stays laminar even at Re numbers of several hundreds. This is typical both to microjets and macrojets. At the next stage, according to visualizations and anemometer measurements, the coordinate of laminarturbulent transition L in the submerged jets was determined at variations of Re number. Our experimental data was compared with data of other authors. The length of the laminar zone in the round jets is twice as large as in the plane jets. Our experiments for the plane jet agree with data of Gau C. et. al. 2009 (nozzle height is 50-360 μm). Data for the round jets of different authors diverge significantly.